1. Field of the Invention
The present invention relates to a sheet material discrimination apparatus for allowing an impact force applying member to collide with a sheet material and detecting an impact force through a sheet material by a detecting unit such as a piezoelectric element, and more particularly, to a technology of improving stability of an output of a piezoelectric element to enhance a discrimination accuracy.
2. Related Background Art
In an image forming apparatus adopting an electrophotographic system, an image forming apparatus adopting an ink jet system, a printing apparatus, and the like, it is preferable to automatically discriminate a sheet material to be processed and adjust an image forming condition, a treating condition, a conveying speed, and the like. Then, various sheet material discrimination apparatuses for automatically discriminating a sheet material have been proposed.
Japanese Patent Application Laid-open No. 2004-026486 discloses a sheet material discrimination apparatus for detecting an impact force caused when an impact force applying member is allowed to collide with a sheet material by a piezoelectric element. In this case, a sheet material is discriminated by detecting a voltage output generated when the piezoelectric element receives an impact force to be deformed by bending.
Japanese Patent Application Laid-open No. 2005-024550 discloses a sheet material discrimination apparatus for allowing an impact force applying member to collide with a sheet material to detect an impact force through a sheet material by a piezoelectric element. In this case, the piezoelectric element is sandwiched between an impact force receiving member and a cushioning material, and a compressive force due to the impact force received by the impact force receiving member through the sheet material affects on a whole surface of the piezoelectric element. The cushioning material absorbs the impact force received by the piezoelectric element to prevent a noise and a vibration of a casing.
U.S. Pat. No. 6,397,021 discloses an image forming apparatus for forming an image on a sheet material. In this case, an image is formed in an image forming part, and then temperature of a heat roller of a fixing device for fixing a transferred toner image on a sheet material is detected to control the temperature of the heat roller based on the detected temperature.
In the sheet material discrimination apparatus disclosed by Japanese Patent Application Laid-open No. 2004-026486, a piezoelectric element is positively deformed by bending due to an impact force of an impact force applying member to obtain a large output. However, the piezoelectric element is allowed to be arbitrarily deformed by bending, whereby the piezoelectric element may cause a bending vibration due to an impact force and a contact with the sheet material. Due to the bending vibration of the piezoelectric element, a complicated vibration mode is formed in a plane of the piezoelectric element, to thereby generate a large spike noise which is superimposed on the output of the piezoelectric element. As a result, in a simple output processing for simply detecting peak values and sorting the obtained values, a measurement result high in reproducibility is not be obtained, and a resolving ability for discriminating a sheet material is lowered.
In the sheet material discrimination apparatus disclosed by Japanese Patent Application Laid-open No. 2005-024550, because bending and deformation are constrained by an impact force receiving member, a bending vibration of the piezoelectric element is less likely to be caused. However, when a height of a detecting part, which includes a cushioning material by reducing a width of the impact force receiving member, is to be reduced, a bending rigidity of the piezoelectric element becomes insufficient, whereby a vibration is more likely to be caused. An output of the piezoelectric element becomes a value corresponding to a combined stress for every deformation such as a slip, shearing, compression, and bending of the piezoelectric element. Therefore, in view of extracting a signal component with a good SN ratio in a single mode, it is considered that a component of another mode is included as a noise component.
Therefore, by focusing on a compressing transformation of a stress of a piezoelectric element, the present invention has been made with a structure in which a detecting unit for extracting only a compressional component is provided, and a stable output with less noises is obtained.
Extracting only a compressional component according to the present invention includes not only a case of extracting only compressional components (100% compressional component) but also a case where a compressional component is predominant among the extracted.